It depends whether I'm interested or not! In pressure exam situations I can concentrate for 2 hours or more. Usually however, the exams we're given in England are so easy that I don't need to concentrate for very long. Don't know if you'd heard about this, but our exam system is so shite the government is replacing the whole lot. In several of my 1 hour 20 minute maths exams I only worked for 45 minutes. And I got an A...

yeah if you just stopped you'd be back to normal quite quickly, however if you were smoking a pack a day for 15 years and then quit, it would be a couple of decades before the risk of cancer and heart attack go back to the level in non-smokers.